1. Field of the Invention
This invention relates to surface acoustic wave delay lines having a series of tap transducers which may be selected to impart a desired delay to a signal propagating along the delay line and, more particularly, to the inclusion of additional electrodes interspersed among the foregoing series of transducers to prevent the generation of reflected waves which degrade signal quality.
2. Description of the Prior Art
Surface acoustic wave (SAW) delay lines are widely used in signal processing operations. Typically, such delay lines are composed of a crystalline material which undergoes mechanical strain in response to an impressed electric field. The delay lines are constructed with a series of paired electrodes in contact with a surface of the crystalline material, wherein each paired electrode in combination with the crystalline material may be regarded as a transducer for converting an electric field into an acoustic wave which travels along the delay line, and for converting energy of the acoustic wave back into electric energy. The primary mode of propagation of the acoustic energy is in the form of a surface wave which travels along the surface to which the electrodes are appended. The acoustic wave travels at a predetermined speed between electrodes of a transmitting transducer to electrodes of a receiving transducer, the surface wave imparting a delay to the propagation of signals between the transmitting and receiving transducers dependent on the distance between the transmitting and the receiving transducers.
In the case where only one transmitting transducer and only one receiving transducer are employed, the SAW delay line serves the function of providing a fixed delay. However, delay lines are frequently built with additional electrode structures arranged serially along the delay line to permit a user of the line to select any one of many possible amounts of delay by connection of external electric circuitry to any desired pair, or taps, of the transducers. Such a delay line provides the function of a variable delay. A multiply-tapped delay line can also be employed as a transversal filter in which a signal enters the delay line via a transducer at one end thereof, and is extracted as a set of signal replicas from each of a set of taps whereby the signal replicas have different delays. As is well known, each of the delayed replicas is multiplied by a suitable scaling factor whereupon the signal replicas are summed together to form an output signal of the transversal filter.
A problem arises in the case of delay lines having a series of tap transducers for producing the function of adjustable delay, or the function of a transversal filter, due to the reflection of surface waves from electrodes disposed on the surface of the crystalline material. Such reflections result in the generation of waves propagating in both directions throughout the crystal and the extraction of an output signal at a selected output transducer, wherein the output signal is degraded by the presence of unwanted signal components produced by the foregoing reflections. Various techniques have been employed to solve the problem, such as the use of absorbant material, grounding of unused electrodes, and encasing the delay line in a metal case. However, these techniques by themselves have not been fully successful because they fail to address a fundamental problem, namely, the generation of the reflections at the individual electrodes due to discontinuity of the transmission medium.